Process for reduction of ores to metals, alloys, interstitial and intermetallic compounds



Aug. 20, 1963 c sHEER ET AL 3,101,308 PROCESS FORVREDUCTION 0F'. GRES TO METALS, ALLOYS, INTERSTITIAL AND INTERMETALLIC COMPOUNDS Filed OCT.. 11, 1960 R52-pac 770A/ FPO/w Ms mL OX/Dg 0M Y Gef/era or Termina/S PROCESS FOR REDUCTION F DRES T0 METALS, ALLOYS, INTERSTIIIAL AND INTERMETALLKC COMPGUNDS Charles Sheer, Teaneck, NJ., Samuel Korman, Hewlett, N.Y., and James 0. Gibson, New Providence, NJ., assignors to Sheer-Korman Associates, Inc., New York, N.Y., a corporation of Delaware Filed Oct. 11, 1960, Ser. No. 61,870 7 Claims. (Cl. 20d-164) This invention relates to a process for the oarbothermic reduction 'of ores concentration of metal compounds particularly oxide compounds for the production of metal carbides, by the application lof a hierarc in a new and unique manner. In this :application the term hierarc is used to dene a particular form of `arc discharge whose gas discharge mechanism yand basicproperties are distinct from the conventional [form of arc discharge commonly used today in electrochemistry.

The hierarc is established whenever a gas or vapor is caused to flow in appreciable amounts into the conduction zone of the are from that lportion of the surface of the anode which is enged in receiving current from the arc, i.e. which is serving as the positive electrode terminus of the discharge. Thus, inthe vicinity of the electrode :boundary, the gas or vapor flows against or in opposition to the flow of current, thereby exerting some form of Iaerodynamic inuence on the stream of electrons and ions comprising the arc current. i

The exact nature of this fluid mechanical gas-stream interaction with the arc current stream is :as yet unknown. However, its effect fon the characteristics yof the [arc dislcharge are quite profound. In particular it causes a radical shift in the manner in which the energy of the `arc is concentrated.

For example, in the conventional arc discharge, the major portion (80-90%) of the energy input is dissipated uniformly Within the conduction column between the anode and cathode, and escapes `out the cylindrical sides of the column into the surrounding atmosphere by radiation, thermal conduction, and diffusion. The remainder of the input energy (l0-20%) is transferred to the tWO electrodes at the current boundaries.

In the hierarc, however, the bulk of the energy dissipation l(6G-75%) is shifted to a very` small region adjacent to the electrode boundary from which the gas is flowing. This energy focusing property results in the establishment of `a much more highly concentrated energy zione within the discharge itself than can occur in any other lcnown form `of arc discharge. This in turn provides the basis for the achievement of either much higher temperatures for more rapid and efficient transfer of energy to the gas stream flowing through the zone and to the subjacent electrode boundary, or both, depending on easily controllable operating parameters. In such a situation only a minor fraction off the total energy dissipated now escapes the discharge site without first havinggbeen trans-v ferred to said ygas stream and electrode. Y

The inventors have discovered that the energy focusing property is directly related to the passage of gas into the arc from the current-receiving electrode boundaries. rl'lhe eect becomes very pronouncedwhen the gas ow occurs in amounts sufficient to affect the electrical charge distribution in the conduction column. Under these conditions it is believed that lthe ow of gas particles is comparable in amount to the opposite flow of electrons which constitutes the arc current. Moreover, the change in varo characteristics, when the mode of operation changes 'from the conventional are to4 that of the hierarc, :occurs quite suddenly `as the iiow rate of gas from the electrode boundary approaches this value.

` United States Patent O ous stream of vapor.

y, 3,101,308 Patented Aug. 20, 1963 "ice Since theelfect is more easily established at the anode than at the lcathode, the inventors have preferred to work with gas flow from the positive terminus lof the discharge. The description of practical results below will therefore mention only the yanode in connection with this phenomenon. It is to be understood, however, that either electrode may be used, :or both, and that the arc may be operated either `on D.C. or on single or polyphase A C., duritng the conduct of this process. In summary, the hierarc is :an arc discharge in which the bulk of the energy is concentrated in a very small zone adjacent to the electrode boundary of the discharge by reason of the flow of gas or vapor from said boundary when the rate of iiow has been properly adjusted to the arc current.

In our first application of the hierarc to metallurgy t We fabricated ya carbon electrode containing the tore to be reduced, and we operated it as an anode in an electric arc. At a certain value of current density it was hound that the surface lof the anode reached a point where there was projected into the arc lfrom the anode surface a copi- The latter consisted of vaporized anode material which issued'from the arc at very high temperature and in higlh-lyionized form; rThis stream We have called the tail flame. 'This material formed a high speed jet in which the constituents of the anode were present in elemental form. The process was operated in an atmosphere of chlorine to prevent the metals from reoXidizing las they cool.

This effect is very pronounced when the gas How is in substantial amounts and it appears to reach its when the iiow of gas particles from the anode is comparable to the flow of electrons which 'constitute the arc current. Moreover this transition from the common arc to the hierarc is quite sudden.

Upon lirst striking the arc, the passage of the current to the electrodes must lirst heat up the electrode surfaces yat the contact interface. But then suddenly, when 'the anode face reaches the boiling po-int lof the anode material, the copious vapor stream bursts out, and the high concentration of energy transfer to the anode face characteristic of the herarc is established.

There is moreover a marked maximum of reaction rate at the point Where the number Iof particles of the anode material moving in the llame is commensurate with the opposing flux of electrical particles comprising the discharge current. That is the number of molecules per second leaving the anode per square centimeter of active anode crater should .be lof the same order `orf magnitude as the number of current-carrying electrons per second entering the same area.

In this iirst embodiment therefore the jet of material projected into the arc Was obtained by utilizing the energy focusing property of the hierarc to vaporize the material of the anode itself. The anode face therefore was brought to the temperature of vaporization of the ore and carbon in the anode.

We also devised a different form in which the vapor necessary to establish the hierarc effect could be introduced into the anode sheath of the arc by using a porous o r finely perforated anode and injecting the vapor through the pores of the anode. In this way the material to be reacted upon could be introduced into the arc as a gas and subjected to the intense energy concentration near the anode characteristic of the hierarc. At the same time the passage of the gas through the -pores of the anode serves to keep the porous anode relatively cool, thus preventing the consumption of the porous anode structure itself. In this modification the external gas, rather than the vvapor ofthe anode, provides the iuid mechanical interaction with the` arc discharge which causes the arc operation to shift to the hierarc mode. The porous anode,

which is not consumed during operation, serves merely as a current collector.

Now the inventors have discovered still a third metthod of establishing a hierarc. This consists of promoting a chemical reaction at -the surface of the anode such that at least one of the products is in the gaseous phase and issues from the active surface at a rate relative to the arc current suicient to initiate the hierarc.

One of the distinctive features of this modification is lthatl it no longer involves the requirement that the material of the anode at the face of thecrater be raised to the Vaporization point. This results from the Ifact that Vthere `are a large number of chemical reactions in which refractory products are formed which can occur between materials ofthe anode in the solid or liquid state, and which produce one or more products in the gaseous state. Furtherthese reactions may be caused to proceed at practical rates even though the reacting `partners are heated to temperatures considerably below the boiling points of the refractory products.

' Finally, the experiments of the inventors have demonstrated that ythe rate at which the product gas can be evolved at such lower temperatures is suicient to initiate and sustain the hierarc mode of operation, even when the arc current -is too low to cause any' appreciable vaporization of the anode constituents.

The accompanying diagramillustrates how the process has been carried out in practice. v

' Infthe drawings the refractory me-tal to be recovered is rproduced in liquid form, and as here shown, the energizing gas is CO. This is produced by the reaction between the ore,hparticularly the oxide ore, and carbon at the anode crater.

In this invention the energy applied to the anode is controlled or limited in such' a manner that the vaporization point of at least one of the reaction products is not reached. This control can be attained by varying the rate offeed of current to the arc, or by varying the rate of rotation of the electrodes.

yReferring now to the iigure, the numeral 1t) represents an electrode fabricated, for example, of columbium oxide with a stoichiometric quantity of carbonsuch as to reduce the metal to metallic form or to a carbide, and discharge the oxygen as CO gas. The numeral 11 is a brush or slidable contactor of some suitable kind, to connect the positive side' 12 of a source of power with the electrode 10. The numeral 13is a cathode, connected by a brush 14 with the negative side 15` of the source of power.

. 4 Y v principle to transfer energy to a reacting mixture such as, for example an oxide ore and carbon in order to produce either the metal or a metal carbide. Which of these two products is produced can easily be determined by adjustment of the carbon stoichiometry. This is illustrated in the following equations:

(where Me=any metal, assumed in this example to bel divalent).

In Example 1 the product will be a metal and carbon monoxide, while in Example 2. the product is the metal carbide plus carbon monoxide. In both cases the reaction during operation was found to be relatively non-critical as regards promoting the reduction. However, it was found that better control ofthe reaction and a purer prod-uct could be obtained by reduced pressure. For this reason the inventors prefer to operate at pressures in the f range from `0.01 to l0 mm. of mercury although useful results may be achieved outside this range of pressures. One of the important requirements for this process is that the reaction nate be sufficiently rapid so that (a) enough `CO lgas is generated to maintain the hierairc en- The anode face 1'6V is of a largerv area than the crater of thearc which is to be struck upon it, and as here shown the cathode is mounted to contact the anode face orf center, preferably at a downwardly inclined angle above the center. s l

When the cathode rst contacts the anode face the current owing through the resistance of the contact raises the temperature of the local area of the anode face up to the point of thermal-reduction ofthe metal oxide of the ore 'with the carbon present. This reaction produces carbon monoxide gas, which allows into the arc conduction column and interacts with the current, thus establishing the hierarc as soon as the contacts are separated. Thereafter, the hierarc eifect may be maintained by the product carbon monoxide gas liberated continuously by the reduction reaction occuring at the anode surface, while the molten metal product drips 01T the anode as shown.` It may be recovered as beads or round shot, or it canbe caused to .fall into a molten bath.

The anode may be rotated about its axis, during the will occur, in an atmosphere of hydrogen or hydrocarbon.

The present invention is'based on the use of the hienarc ergy distribution and (b) the reaction is essentially complete before any of the constituent materials leave the anode and enter the arc'. The reason for the ii-rst of l these requirements is obvious view of the previous disoussion. 'IheV second requirement stems from the fact that if the rneaction is incomplete before the reacting components reach the arc crater the end product will be contaminated with unreacted starting materials. s

In order to achieve the desired condition it is essential that the anode surfaceV at the electrode-arc :boundary be maintained at an appropriate temperature within a well dened range.

The lower limit of this range is prescribed by the requirements (a). and (b) above. The existence of an upper limit was discovered during the experimental investigations of the inventors and arose for the following treason: When the surface temperatutre at the anode crater is raised .beyondV a certain point (which point may still be well below the vaporization temperature of the constituents) the thickness of the .section of the electrode behind the anode crater which 1s hot enough to promote the carbothermic reduction increases. Consequently CO" gas is generated at some depth within the body of the electrode. At sutiiciently high temperatures the gas may be generatedA at a point -too far fromthe crater surface to all-ovv it to escape by `diffusion at the frate at Which it is generated.

When this occurs, the pmessure of said product gas within the electrode begins to build up until it either shatters the electrode or splatters the molten off the anode before the reaction is completefait thatV point, thus interruptmg the process. The upper limit of surface temperatures during operation therefore is dictated'by the pairticular characteristics of the .ore being used and the product being produced. This obviously follows from the fact that the reaction rate for canbothermic reduction is a function of the temperaturey as well as the reaction rate constant for the particular materials involved. This constant 4is different for each ore, and for the same ore, is idiiferent for production of the metal than for the production of its carbide.

However, once the starting materials and products are specified it is fairly simple to determine the practical limits of operation as regards surface temperature. All that is required is first to adjust the conditions of operation so that the surface temperature is high enough to produce the plasma tail flame of CO gas, which is" easily visible and which is a sign that hierarc operation has been achieved, and secondly to increase thertemperature beyond this point to the degree necessary to prevent unreacted :ore from appearing in the product. This establishes the lower limit of surface temperature during operation.

The upper limit of the practical operating range is then determined -by increasing the surface temperature until spalling of the electrode o-r splattering of the molten film q occurs. The practical working range lies between these two limits.

It should be emphasized that, in contrast to our previous application, the reaction zone does not occur anywhere in the arc itself, but rather in a thin film of materia'l across the tip of the elect-rode subjacent to the are crater. Despite this fact, operation of the arc in the hierarc mode is essential to achieve the practical results the inventors have observed. 'IThey have noticed invariably that kwhen the prevailing mode off operation is that of the conventional arc, ie. when the rate of CO evolution from the surface is too low to cause the hierarc focusing effect, the rate of production becomes very small relative to the power expended, and the process becomes highly inefficient. rDhus, when the latter condition obtains, the bulk lof energy is not transferred to the reacting materials and is therefore not used in promoting the reaction. Instead it is wastefully dissipated.

Concurrently the inventors have found that the degree of contamination .of the product, when the conventional type of arc loper-ation is used, is far -greater than when the hierarc is established.

A second important feature of this new technique is the discovery of the inventors that the optimum range of temperature for promoting the reaction in the manner described is generally such that one lor more of the product materials involved is in the molten state. For example, referring to Examples l and 2 above, itis found that the metal or metal carbide exists in the molten state when the process is conducted within the optimum range of surface temperatures. Moreover, this range of temperatures is sufficiently wide so that an operating tem` perature may be selected at which the molten film at the anode face exerts negligible vapor pressure. At thisl temperature therefore the tip of the electrode is always covered with a thin film of molten metal or metal car` bide through which the CO `gas can easily diffuse without significant losses of the desired product.

As `the reaction proceeds the thickness of the molten film, which adheres to the anode face initially by surface tension, becomes thick enough so that it agglomerates in the form of -a :droplet which detaches itself from the anode surface and falls under the action of gravity. In Athis manner the product automatically and continuously leaves the reaction zone so that `the thickness of the molten film remains essentially constant while the reaction proceeds indefinitely. Moreover, the length of fall provided for in the reaction chamber is sufficient, the droplet of metal or metal carbide will freeze before it strikes the bottom of the reaction chamber. Once frozen to the solid state the metallic bead is relatively inert and does not become contaminated by contact with the chamber wall.

This manner of operation has several extremely valuable properties; first, it permits the reduction reaction to be carried on continuously; secondly it provides for the removal of the product from the reaction zone in such a way as to avoid completely the possibility of any back reaction, so that quantitative yields can be obtained; and thirdly, by maintaining the proper surface temperature, the reduction reaction can `be made to go to completion before the droplet agglomerates and falls off the surface. Furthermore, if the droplet is allowed to freeze before striking the bottom, the product may be withdrawn from the chamber without ever having `come in contact with the walls of a containing vessel while in the reactive molten state. This feature completely avoids one of the more baiing and difficult practical problems encountered in the field of metallurgy. This problem relates to the problem of the `Crucible required to contain the materials which is now universally used in competitive conventional techniques. In such processes vessel walls are invariably attacked by many molten metals or carbides especially for such elements as columbium, titanium, uranium, etc. When these materials come in contact with the container Walls in the molten state, the melt attacks the crucible walls until the Crucible is destroyed, and at the same time the product becomes contaminated with container material.

In the present process this problem is completely eliminated and the product is produced in the form of round shot of especially high purity. l

A further advantage which may be noted at this point is that when the process i-s conducted at low ambient pressures, volatile impurities are expelled from the molten film while the material resides on the tip of the electrode. Thus, some of the purifying action characteristic of vacuum melting is secured by this technique simultaneously with the reduction process and without extra cost.

`For example, in the production of columbium the following purification took place:

In ore: Ppm. Zirconium 1000 Si 1000 In product:

Zirconium 200 Si It has also been found possible to regulate the size of the spheres of metal or metal carbide produced in this process by la slight variation in the conditions of operation. This is done by varying the operating surface temperature within the operating range described above.

At the lower limit of this range the liquid product accumulates in the form of relatively large droplets since the pressure of CO gas is produced sufiiciently low so that it does not disturb the dripping process. As the surface temperature is raised slightly the pressure of CO gas within themoltenfilm increases, and we have found that this has a tendency to cause the molten droplets to detach from the surface with` smaller sizes.

At the upper extreme of the `operating range, before the onset of excessive splatter or spalling within the electrode, the particles have` been observed to blow off in the form of very small droplets in the fashion of a liquid atomizer. Thus, using a 3A" diameter anode it has been found possible to vary at will the size of the spheres produced from 5/16 diameter down to as low as -325 mesh.

In the practical application of this process the inventors have varied two parameters in order to facilitate the achievement of the optimum temperature at the anode surface. One of these is the total arc current for a given electrode diameter, and the second is motion of the anode crater on the anode surface. Since in this modification of the process it is no longer necessary to cover the entire -tip of the electrode, the arc current can be reduced until the arc terminus is considerably less than the diameter of the tipi. i

The inventors have `found that if the electrode is rorated at the proper speed so that the arc terminus sweeps out `the total electrode surface area, 'the entire electrode may be consumed while at the same time the average surface temperature obtaining over the surface may easily be adjusted over a wide range. i

Typical examples of this process which have been worked out in the laboratory for each type of product are 'the following:

(A) Production of Metal For thisA purpose the carbothermic reduction of colum- V biurn oxide to produce columbium metal has been chosen.

The reactionin accordance with the equation is Yas follows: l

f Cb2o5+5c 5co+2cb 3000l grams, -100 mesh calcined Cb2O5 356.7 grams, -325 mesh coke flour (eg. 99% C coke of the Wilson Coke & Chemical Co.)

149.1 grams liquid pitch paint (e.g. eternium paint of the Allied Chemical Corp.)

The columbium `oxide and coke were dry mixed -for one-half hour in a laboratory-miX-muller. The liquid pitch paint wasv slowly added and the mixing continued for an ladditional one-half to rthree-quarters of an hour. The plastic mix was loaded into the cylinder of an extrusion press, vde-aired, and extruded into convenient lengths ,at an extrusion pressure of 2750 p.s.i. The extruded electrodes Were inserted into graphite tubes which were stacked in a stainless steel tray, covered wi-th coke ilour, and (baked. During the baking operation, the loaded tray was inserted'into anY electric furnace and heated at a rate of 100 C. per hour to 900 C. and held labout four hours at 900 C. to complete the baking process. The baked electro-des were straight, Ysmooth and hard, with excellent compress-ive strength. Electrical resistivities in the order of .05 ohm-centimeters and densities in the order of 4.23

` grams per cc. were achieved.

These electrodes were inserted into the feed mechanism of the arc chamber andthe vessel was then pumped down to the vorder of 1000 microns pressure or less. F[he arc was then struck by contacting the anode with the cathode and retracting. the cathode to its operating position.

Typical values of the operative conditions are given in the following:

The best results in the laboratory `were attained by using a 2 diameter homogeneous cathode of the same composition as 'the anode which was 3A diameter.

The process of making colnnibium metal from its oxide produced the :metal in the norm yof :spheres 1A to 5/16'.' diameter, of 99.8% purity. t

(B) Production of Metal Carbide This is illustrated by the production of uranium monocarbide from uranium dioxide. The equation for this reactionis given specifically fby:

The procedure for fabricating electrodes with uranium dioxide and carbon is essentially the same as that de- Y scribed above for columbium, except for the proportions of ingredients. 'For example, We have used the formula tion as follows:

1135 grams, -l00 mesh uranium dioxideV Y' 105.5 grams, 325 mesh coke our v 149.1 grams, liquid pitch paint 4 v l Y For best results it was also found desirable to alter the baking schedule slightly whereby the green electrodes were tired by `raising the furnace temperature initially at a rate of 50 C. per hour until a temperature of 700 C. was reached. Subsequently the temperature was raised at a rate of 100 C. per hour until the maximum temperature 4of 900 C. was achieved; the temperature was then held at 900 C. for four hours. I'

The arc burning procedure followed that described for columbium metal except fior the precise values of tlhe oper'- ating parameters. Values used in the laboratory for producing UC are given in the following: i

Electrode diameter inches-.. S Carbon content of baked f electrode percent..- 11.8 to 12 Arc current Iarnperes-.. 60 .to 100 Arc Voltage volts 40 no 60 Ambient pressure V.. microns 200to1500 Anode rotation rate r.p.m Sto 20 a hierarc between Ia cathode Iand said anode, while limiting the temperature of the anode face at the arc crater to a point between the boiling point and the melting point o'f the metal product, wherebythe said metal product `accumulates in molten form upon the face of the anode until it drips olf, while the other products are carried into the arc as vapors.

2. A process according to claim 1, in which the ternperature of the crater is controlled by moving the anode face transverse to the arc terminus so that cooler anode material is continuously presented tothe arc. Y t

3. A `process according to. claim 1, in which the anode is in the form of a cylinder larger than the arc crater, which cylinder is rotated about its laxis and the arc cra-ter `is on the end of the cylinder on one side of the axis. v 4. A process according to claim 1, in which the anode is in theform of a cylinder larger than the arc crater,

which cylinder is rotated about its axis and the arc crater is on the endl of the cylinder on one side of the axis, by varying the current fed to the -arc relative `to the speed of rotation. v Y

5. A process of conducting and controllingv a hierarc to recover values from an ore which comprises establish- 4ing a-hierarc between va cathode and lan anode containing the ore and carbon, lsaid anode having greatersurface are-a than the hierarc crater to be established,` striking such an arc with said anode to form a crater while continuously moving said anode lto cause the crater to change its position continuously, to maintain the temperattire ot the crater below the vaporizing temperature vof the metal values. f

6. The process of producing amet-allic-product in the form of `spherical shot which comprises fabricating an anode comprising a mixture of carbon with metal-containing ore, establishing `a hierarc with said anode to form a crater while continuosulymoving the position of s-aid crater so as to maintain the'tip of the anode between the melting and boiling points of the product sought.

7. The process of reducing an ore which comprises forming an electrode of said ore with a quantity of car- 9 10 electrode as anode n an inert gas, whereby said ore is References Cited in the le of this patent reduced at the anode face by the heat of (the arc, While Johnson et aL: Arcs in ,Inert Atmospheres and maintaining the temperature at the anode face above the Vacuum (1956), Pages 25 and 26- melting point but below the vaporizaton temperature of The High Intensity mectric Arc and `Its, Application to the product. 5 Process Chemistry, May 25, 1956, pages 2O and 22. 

1. A PROCESS OF RESUCING AN OXIDE ORE WHICH COMPRISES FORMING AN ANODE OF CARBON AND A STOICHEMICAL QUANTITY OF ORE TO PRODUCE THE METAL PRODUCT DESIREDM MAINTAINING A HIERARC BETWEEN A CATHODE AND SAID ANODEM WHILE LIMITINF THE TEMPERATURE OF THE ANODE FACE AT THE ARC CRATER TO A POINT BETWEEN THE BOILING POINT AND THE MELTING POINT OF THE METAL PRODUCT, WHEREBY THE SAID METAL PRODUCT ACCUMULATES IN MOLTEN FORM UPON THE FACE OF THE ANODE UNTIL IT DRIPS OFF, WHILE THE OTHER PRODUCTS ARE CARRIED INTO THE ARC AS VAPORS. 